The membrane currents underlying cardiac action potentials The Nat current(INa): is responsible for the rapid depolarizing phase of the action potential in cardiac muscle cells and in Purkinje fibers The Ca current(ca): is responsible for the rapid depolarizing phase of the action potential in the sa node and av node; also triggers contraction in all cardiomyocytes The k* current (k): is responsible for the repolarizing phase of the action potential in all cardiomyocytes The pacemaker(Funny) current(f): is responsible, in part, for pacemaker activity in SA nodal cells, av nodal cells and purkinju fibers; is mediated by a nonselective cation channelThe membrane currents underlying cardiac action potentials • The Na+ current (INa): is responsible for the rapid depolarizing phase of the action potential in cardiac muscle cells and in Purkinje fibers. • The Ca2+ current (ICa): is responsible for the rapid depolarizing phase of the action potential in the SA node and AV node; also triggers contraction in all cardiomyocytes. • The K+ current (IK ): is responsible for the repolarizing phase of the action potential in all cardiomyocytes. • The pacemaker (Funny) current (If ): is responsible, in part, for pacemaker activity in SA nodal cells, AV nodal cells and Purkinju fibers; is mediated by a nonselective cation channel. 15